The present invention relates to a base band unit of a transmission node cluster of transmission nodes in a wireless cellular network, to a wireless cellular heterogeneous network comprising at least a transmission node cluster, and to a method for providing generic hierarchical precoding codebooks in a wireless cellular heterogeneous network.
In the present patent application, the following abbreviations are used:
BBU Base band unit
CAS Central Antenna System
CQI Channel Quality Indicator
CB Codebook
CPM Cluster Precoding Matrix
DAS Distributed Antenna System,
MTP Multipoint-To-Point
MIMO Multiple Input Multiple Output
OM Operation Mode
PTP Point-To-Point
PM Precoding Matrix
PMI Precoding Matrix Indicator
RI Rank Indicator
RRU Remote Radio Unit
SNR Signal to Noise ratio
SINR Signal to interference and noise ratio
TN Transmission Node
TNC Transmission Node Cluster
UE User Equipment
A wireless cellular network can comprise a plurality of cells, wherein each cell comprises a base station, and a BBU. In DAS systems RRUs with one or more antennas are provided to enhance the coverage and capacity in the wireless cellular network. In a DAS the RRUs can be connected to the base station via a high bandwidth and low latency link. In a cellular wireless network the RRUs are used in a DAS to provide more uniform coverage, reduced outage and higher throughputs especially in shadowed and indoor locations. A DAS can be combined with a MIMO communication system by using the RRUs as a distributed antenna array and/or equipping the RRUs with multiple antennas. In a homogeneous wireless network the cells of the wireless cellular network are of similar shape and size. In contrast, in a wireless cellular heterogeneous network cells of the network are of different size and type.
In a MIMO system which can be provided in a DAS, more than one transmission antenna can be used to send a signal on the same frequency to more than one reception antenna. Conventional cellular networks generally provide a best service under line of sight conditions. In a MIMO system rich scattering conditions can be exploited by signals which bounce around in the environment. Under rich scattering conditions signals from different transmit antennas can take multiple paths to reach a UE at different times. A radio frequency signal path from a transmitting antenna to a receiving antenna is gradually weakened, while interference from other radio frequency signals reduces the SINR of the signal. In addition, in crowded environments, the radio frequency signal frequently encounters objects which alter its path or degrade the signal. A multiple antenna system can compensate for some of the loss of the SNR due to multipath conditions by combining signals that have different fading characteristics.
To achieve throughput gains where the signal-to-noise ratio is relatively high, a wireless cellular network can use a MIMO technique called spatial multiplexing. In spatial multiplexing each transmitting antenna sends a different data stream to a multiple receiving antenna. These data streams are then reconstructed separately by the UE.
With spatial multiplexing, one can transmit different signals at the same time over the same frequency. With spatial multiplexing it is possible to increase the transmission data rate. To do this, the data is divided in two separate streams, wherein the streams are transmitted independently via separate antennas or groups of antennas.
Each set of data sent through the antennas in spatial multiplexing operation is called a layer. In spatial multiplexing, rank refers to the number of data streams transmitted over the same time-frequency resource, corresponding to the number of layers.
A cellular wireless network can be operated in a closed loop or an open loop mode. It is possible that a base station communicates with the UE in an open loop when the UE is moving too fast to provide a detailed report on channel conditions on time for the base station, to select a PM. In open loop operations, the base station receives only minimal information from the UE.
In a closed loop operation of the wireless network the UE can analyze the channel conditions between a transmit and receive antenna including the multipath conditions. The UE then provides a RI as well as a PMI which determines the optimal PM for the current channel conditions. Finally, the UE can provide a CQI given the RI and the PMI rather than basing the CQI only on the current OM. This allows the base station to quickly and effectively adapt the transmission of data to the current MIMO channel conditions. Closed loop operation of the wireless network is particularly relevant for spatial multiplexing where the MIMO system offers the greatest throughput gains.
FIG. 1 shows a diagram for illustrating spatial multiplexing as employed in a conventional wireless cellular network. Spatial multiplexing works by creating separate data streams on multiple antennas. With spatial multiplexing, independent data streams can be transmitted simultaneously on the same frequency resource by mapping them to so called spatial layers. The number of spatial layers is the same as the rank, R, of the precoding matrix used for data transmissions.
As shown in FIG. 1 in a multi layer transmission, data arriving from a higher level process comprises codewords. Each codeword is then mapped onto one or more layers. Each layer is then mapped onto one or more antennas using a precoding matrix.
To determine how to map the layers to antenna the complete N×M dimensional signal space is evaluated. For the layers the rank R of the N×M Matrix is decisive.
The PMs used by the base station are stored in a so-called CB. Accordingly, a CB comprises a set of PMs used for precoding in a downlink data transmission between a base station and a UE.
In a wireless cellular heterogeneous network the cells are of different type and size and comprise macrocells, microcells as well as picocells. In a distributed heterogeneous antenna setup a UE in general experiences a different channel gain to each distributed RRU of the heterogeneous network which may be equipped with multiple antennas.
Multiplexing within a wireless cellular network is a possible way to increase capacity and coverage in the wireless network. This can either be achieved by using massive, distributed antenna configurations or by introducing small cells into a macrocellular grid of the wireless network. However, the introduction of more cells within the wireless cellular network using the same frequency does at the same time introduce more interference among neighboring cells.
In particular for a DAS all RRUs can be connected to a BBU using for instance optical fibers. In a closed loop operation the UE does report feedback information on the channel conditions in terms of a PMI, a CQI and a RI. In a conventional wireless cellular network the precoding CBs comprising the PMs are intended for a CAS with a predetermined maximum number of transmit antenna ports. Accordingly, in conventional wireless cellular networks precoding CBs are only considered for CAS and thus lack a suitable definition of CBs for a DAS. The same holds for a heterogeneous network where precoding is treated in each BBU or TN within the heterogeneous network. In contrast to a conventional DAS where all RRUs are connected to the same BBU feedback and control information must be exchanged between TNs of a heterogeneous network in order to achieve a certain degree of coordination between them.
Accordingly, there is a need for a method and an apparatus for providing generic hierarchical precoding CBs for a wireless cellular heterogeneous network/SUMMARY
According to a first aspect of the present invention, a BBU in a wireless cellular heterogeneous network is provided, the BBU being provided in a TNC of TNs of neighbouring cells of the wireless cellular heterogeneous network, wherein the BBU comprises generic hierarchical precoding CBs, each CB comprising CPMs, and each CPM is provided for a possible combination of active TNs within the TNC.
In a first possible implementation form of the BBU according to the first aspect of the present invention, each CPM provided for a possible combination of active TNs within the TNC is constructed on the basis of PMs associated with the active TNs.
In a second possible implementation form of the BBU according to the first aspect of the present invention or according to the first implementation form of the first aspect of the present invention, the CPMs having the same rank form a generic hierarchical precoding CB for the respective TNC.
In a third possible implementation form of the BBU according to the first aspect of the present invention as such or according to any of the preceding implementation forms of the first aspect of the present invention, the CPM of the TNC is a N×M matrix comprising the PMs of all active TNs of the TNC, wherein N is the number of TNs in the TNC and M is the number of supported global spatial layers in the TNC, with M≦N.
In a fourth possible implementation form of the BBU according to the first aspect of the present invention as such or according to any of the preceding implementation forms of the first aspect of the present invention, a rank R of the CPM of the TNC corresponds to the number of spatial layers that are active in the TNC, with R≦M≦N.
In a fifth possible implementation form of the BBU according to the first aspect of the present invention as such or according to any of the preceding implementation forms of the first aspect of the present invention, a number Z of all CPMs having the same rank and forming a generic hierarchical precoding CB for the respective TNC is given by:
      Z    =                  (                                            N                                                          A                                      )            =                        N          !                                                    (                              N                -                A                            )                        !                    ⁢                      A            !                                ,
wherein N is the number of all TNs in the TNC and A is the number of the active TNs in the TNC, wherein A is equal to R.
In a sixth possible implementation form of the BBU according to the first aspect of the present invention as such or according to any of the preceding implementation forms of the first aspect of the present invention, the CPM of the TNC is stored in a CB memory of a coordinating BBU of the TN within the TNC and is adjustable by the coordinating BBU.
In a seventh possible implementation form of the BBU according to the first aspect of the present invention as such or according to any of the preceding implementation forms of the first aspect of the present invention, the BBU is a coordinating BBU of the TNC and is adapted to select an OM used for a registered UE, depending on at least a calculated signal quality metric of a reception signal at the registered UE and/or reception signals of other UEs registered with a TN of the TNC.
In an eighth possible implementation form of the BBU according to the seventh implementation form of the first aspect of the present invention, the BBU is further adapted to select a hierarchical precoding CB for the selected OM, and to select a CPM within the selected hierarchical precoding CB, depending on the signal quality metric of a reception signal at the registered UE and/or reception signals of other UEs registered with a TN of the TNC.
In a ninth possible implementation form of the BBU according to the seventh or eighth implementation form of the first aspect of the present invention, the BBU is further adapted to offer different hierarchical precoding CBs in different time slots to the registered UE, wherein the registered UE is adapted to select a CPM within an offered hierarchical precoding CB, depending on a signal quality metric of a reception signal at the registered UE.
In a tenth possible implementation form of the BBU according to the first aspect of the present invention as such or according to any of the second to ninth implementation forms of the first aspect of the present invention, each PM of the TN included in the CPM of the TNC is a T×L matrix, wherein T is the number of antennas of the respective TN and L is the number of spatial layers supported by the TN, with L≦T.
In an eleventh possible implementation form of the BBU according to the first aspect of the present invention as such or according to any of the second to tenth implementation forms of the first aspect of the present invention, the PM of the TN included in the CPM of the TNC is adjustable by the BBU of the respective TN.
In a twelfth possible implementation form of the BBU according to the first aspect of the present invention as such or according to any of the preceding implementation forms of the first aspect of the present invention, the CPM of the TNC is stored in a CB memory of the BBU.
In a thirteenth possible implementation form of the BBU according to the seventh or eighth implementation form of the first aspect of the present invention, the signal quality metric comprises an SINR.
In a fourteenth possible implementation form of the BBU according to the first aspect of the present invention as such or according to any of the preceding implementation forms of the first aspect of the present invention, each CPM comprises an associated PMI.
According to a second aspect of the present invention, a wireless cellular heterogeneous network is provided, comprising at least a TNC, wherein the TNC comprises TNs of neighbouring cells and comprises at least a BBU, wherein the BBU comprises generic hierarchical precoding CBs, each CB comprising CPMs, and each CPM is provided for a possible combination of active TNs within the TNC.
In a first possible implementation form of the wireless cellular heterogeneous network according to the second aspect of the present invention, each TN comprises an associated PM and each CPM provided for a possible combination of active TNs within the TNC is constructed on the basis of PMs associated with the active TNs.
In a second possible implementation form of the wireless cellular heterogeneous network according to the second aspect of the present invention or according to the first implementation form of the first aspect of the present invention, the TNC is adapted to support different spatial layers provided for data transmission between at least a TN of the TNC and at least a UE registered with a TN of the TNC.
In a third possible implementation form of the wireless cellular heterogeneous network according to the second aspect of the present invention as such or according to any of the preceding implementation forms of the second aspect of the present invention, the wireless heterogeneous network is operated in different OMs, comprising a PTP-OM and a MTP-OM, wherein in the PTP-OM a UE registered with a TN of the TNC has a transmission link to a single TN of the TNC, to transmit a signal to the registered UE, wherein in the MTP-OM a UE registered with a TN of the TNC has transmission links to a scalable number of TNs of the TNC, wherein through each transmission link an identical signal is transmitted to the registered UE.
In a fourth possible implementation form of the wireless cellular heterogeneous network according to the second aspect of the present invention as such or according to any of the preceding implementation forms of the second aspect of the present invention, a backhaul interface provided between BBUs of the TNC comprises a minimum bandwidth being reserved to exchange messages with control information relating to selected CPMs, depending on the total number of CPMs in all hierarchical precoding CBs of any rank.
According to a third aspect of the present invention, a method for providing generic hierarchical CBs in a wireless cellular heterogeneous network is provided, the wireless cellular heterogeneous network comprising at least a TNC, wherein the TNC comprises TNs of neighbouring cells, and comprises at least a BBU, wherein the BBU comprises generic hierarchical precoding CBs, each CB comprising CPMs, and each CPM is provided for a possible combination of active TNs within the TNC.
In a first possible implementation form of the method according to the third aspect of the present invention, each CPM provided for a possible combination of active TNs within the TNC is constructed on the basis of PMs associated with the active TNs
In a second possible implementation form of the method according to the third aspect of the present invention as such or according to the first implementation form of the second aspect of the present invention, the CPMs having the same rank form a generic hierarchical precoding CB for the respective TNC.
In a further possible implementation form a computer program for implementing the method according to the third aspect of the present invention as such or according to any of the preceding implementation forms of the third aspect is provided.
The generic implementation used by the different aspects of the present invention provide a simple scheme for adapting the spatial precoding for signal transmission to a UE in a heterogenous network and/or DAS allowing to achieve a high throughput on link as well on system level while keeping the required amount of feedback between UEs and the BBU low.